This invention relates to Sm-Co permanent magnet materials and, more particularly, to the improvement of such magnetic materials for permanent magnets wherein the magnetic induction after baking may be maintained greater in comparison with known Sm-Co materials in the prior art.
Sm-Co alloys, in particular such alloys consisting of, by atomic ratio, Sm of 1 and Co of 4.0-5.2, have been used in the prior art for permanent magnet materials.
Sm-Co permanent magnets are classified into two classes, one being a compacted type and the other being a sintered type. In making a permanent magnet of a compacted type, Sm-Co alloy powder is put in an aligning magnetic field and is pressed into a compacted solid body. It is known that a binder is mixed with the powder.
In making a permanent magnet of a sintered type, a compact obtained by pressing the powder under the aligning magnetic field is subjected to a heat-treatment for sintering.
Sm-Co permanent magnets have been obtained which have a maximum energy product (BH).sub.max close to the theoretical upper limit for the Sm-Co alloy.
But such permanent magnets have a disadvantage in that the magnetic properties vary in response to the variation of the temperature.
Variations of the magnetic properties in response to the temperature variation are classified into two types, one of which is called an irreversible temperature loss or an irreversible magnetic induction loss with temperature, and the other of which is called a reversible temperature loss.
The irreversible temperature loss is defined by following expression: ##EQU1## WHERE, Bd.sub.0 is the value of the magnetic induction under the operating condition of a magnet magnetized at a room temperature and Bd.sub.1 is the value of the magnetic induction of the magnet under the operating condition of room temperature after exposure of baking at an elevated temperature for a considerably long period.
The reversible temperature loss concerns the magnetic properties of the magnet after completion of the baking treatment and is the rate of the change of the value of the magnetic induction in response to a temperature variation of one degree within the temperature range below the baking temperature. Thus the reversible temperature loss is defined by following expression: ##EQU2## where, T.sub..alpha. and T.sub..beta. (T.sub..alpha. &lt; T.sub..beta. are different temperatures below the baking temperature, Bd.sub..alpha. is a value of the magnetic induction at the temperature of T.sub..alpha. and Bd.sub..beta. is a value of the magnetic induction at the temperature of T.sub..beta. .
Many reports have been provided relating to these irreversible and reversible temperature losses of the Sm-Co permanent magnet. Those reports teach us that the reversible temperature loss of the Sm-Co permanent magnet generally is within the range of -0.045 .+-. 0.005%/.degree.C which is hardly changed by the variation of the operating point of the magnet. But the irreversible temperature loss considerably varies dependently on the change of the magnetic properties, the operating point and other factors of the magnet.
It is known that a permanent magnet having such a demagnetization characteristic wherein the shoulder portion of the curve within the second quadrant of the magnetic hysteresis curve (or 4 .pi.I - H loop) exist on the greater demagnetizing field, has the lower irreversible temperature loss. And the greater the permeance factor is of the magnet at the operating point thereof, the lower is the irreversible temperature loss of the magnet. For example, after the Sm-Co permanent magnet of a rod shape having a dimensional ratio, or (the length in the direction of the easy magnetic axis of the magnet)/(the diameter of the magnet), of 0.4 and an intrinsic coercive force .sub.I H.sub.C of 20 - 30 KO.sub.e, has been heat-treated at a temperature of 300.degree.C for 3 hours, the resulting irreversible temperature loss is about -10%. But some magnets having a similar dimensional ratio but an intrinsic coercive force less than 20 KO.sub.e, exhibit irreversible temperature losses of values close to -90% after similar heat-treatment.